Cut two 13cm pastry discs and chill them in the freezer for one hour on a lined baking sheet.
Preheat your oven to 180°C.
Slice beetroots into 1mm discs using a mandolin and coat them evenly with warm butter, salt, and pepper.
Layer beetroot discs three deep inside 9cm rings placed within 12cm blini pans.
Cover the beetroot with parchment, replace the ring to trap steam, and heat on the stovetop for 2 minutes.
Bake the beetroot for 12 minutes at 180°C, then cool completely and turn out onto a plate.
Cook sliced onions with butter and seasoning for 40 minutes under a lid, then deglaze with vinegar.
Spread onion jam on pastry discs, apply egg wash to edges, top with beetroot, and bake for 20 minutes.
Whisk olive oil, boiled vinegar, salt, and pepper to create the Jerez vinaigrette.
Serve the tarts glazed with vinaigrette alongside dressed salad leaves and cottage cheese.